drm/i915: don't get/put PC8 reference on freeze/thaw

We already get runtime PM references, and PC8 is now part of runtime
PM, so this is enough.

v2: - Rebase.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>